Saturday, March 15, 2014

Yesterday saw the publication of two new papers: the first of
which is about a new genus and species of bizarre porpoise from the Pliocene of
California, and the second is the published version of my master’s thesis.

The first paper is a collaboration with Rachel Racicot,
Brian Beatty, and Tom Deméré and finally describes the extinct odontocete
informally known as the “skimmer” or “half-beaked” porpoise. The new fossil
phocoenid is named Semirostrum ceruttii, the genus name referring to the
dramatically shorter rostrum, and is also an homage to the half-beak fish, Hemiramphus.
The species name is in honor of Richard Cerutti, longtime field paleontologist
and preparatory for the San Diego Natural
History Museum.
Mr. Cerutti collected the holotype in the early 90’s from the Pliocene San
Diego Formation – I met him during a 2012 visit to the SDNHM.

The holotype skull, earbone, and mandible of Semirostrum ceruttii, and a composite skeletal reconstruction from three specimens. From Racicot et al. (2014); skeletal reconstruction by yours truly.

The new fossil porpoise has a somewhat longer rostrum than
modern phocoenids, and is slightly more delphinid-like than modern porpoises as
well. Most obviously, Semirostrum has a bizarre lower jaw with an
elongate, fused mandibular symphysis that is developed into a laterally
flattened and expanded, paddle-shaped process that juts far beynd the edge of
the rostrum. The teeth in the mandible do not extend past the rostrum, so the
majority of the symphysis is edentulous. The preserved teeth have labial wear
facets, which we interpret as being the result of substrate abrasion – the
observed patterns of tooth wear differ from extant phocoenids in lacking apical
wear facets. When articulated, the wear facets do not match up with occluding
upper teeth – indicating that regular tooth wear does not account for the
observed pattern. We hypothesize that the elongate mandibular symphysis is a
benthic probe, and that Semirostrum pushed its “chin” through the
substrate, with sediment streaming along the lateral sides of the toothrow –
snatching up any burrowing prey that came into contact with the “chin” or
rostrum. In my life restoration, I illustrated Semirostrum as using it’s
mandibular symphysis like a benthic plough, ploughing through the uppermost
layer of the sediment in its search for burrowing invertebrates. The type
specimen consists of a complete skull and mandible with periotic, tympanic
bulla, and postcrania – an absolutely gorgeous set of fossils which I first had
the opportunity to examine on my first visit to the SDNHM back in 2007.

My contribution to the paper was describing fossil material
of Semirostrum from the Purisima Formation. Although the holotype
specimen of Semirostrum ceruttii is from the San Diego Formation,
multiple specimens of Semirostrum have been collected from
contemporaneous sections of the Purisima Formation. In fact, one of the
earliest known specimens of Semirostrum – collected in the mid 1980s by
local collector Wayne Thompson – consists of a pair of fused mandibles. The
specimen still unpublished and is now at LACM, but I was not able to see it on
my last museum visit in November 2013. Material from the Purisima Formation
includes a nearly complete skull and isolated mandible, a partial rostrum, and
a couple of isolated periotics. None of the material is associated – it’s all
scattered material preserved in bonebeds and other inner shelf sediments,
presumably scattered across the seafloor by currents or from drifting
carcasses. Nevertheless, every referred element exhibits morphological features
unique to Semirostrum. Some specimens – including the periotics and the
mandible – are morphologically indistinguishable from and identical in age to Semirostrum
ceruttii. However, the skull and partial rostrum are somewhat older, from
about the Mio-Pliocene boundary; furthermore, the skull exhibits a slightly
asymmetrical facial region – which is a bit more primitive than extant
phocoenids, and Semirostrum ceruttii. For these reasons, we interpreted
this slightly older material as representing an as yet unnamed, slightly older
species, and chose to simply identify it as Semirostrum sp.

Examples of different preservational features on shark teeth (A), odontocete vertebrae (B), auk (Alcidae) humeri (C), and odontocete periotics (D) - specifically, the top periotic is Parapontoporia wilsoni, and the bottom periotic is a referred periotic of Semirostrum ceruttii figured by Racicot et al. (2014:figure 2).

The second paper is the publication derived from my master’s
thesis research at Montana State
University. My master’s thesis
dealt with the taphonomy of Miocene and Pliocene marine vertebrates preserved
in the Purisima Formation of Northern California. I initially got hooked on
taphonomy – the science of fossil preservation – thanks to my undergraduate
adviser Dave Varricchio, who did his Ph.D. on the formation of “Jack’s Birthday
Site”, a multispecies bonebed assemblage from the late Cretaceous Two Medicine
Formation. I took his taphonomy course in fall 2005, and read a few articles on
the taphonomy of modern whales. At that time, I had just returned from my first
summer season of permitted field work in the Purisima Formation, so I was
naturally interested in looking into the taphonomy of the unit. Further piquing
my interest was marine reptile researcher Pat Druckenmiller’s return to MSU to
teach for a year. Pat did his master’s thesis at MSU, where he published the
short necked plesiosaur Edgarosaurus from the Thermopolis Formation near
Bridger, Montana.
Pat had an interest in marine vertebrate taphonomy – and we talked quite a bit
about it.

Histograms of taphonomic characteristics of bones, teeth, and cartilage from different lithofacies of the Purisima Formation. In general, highest energy conditions are on left, lowest energy on right.

As it turns out, the Purisima Formation had already been the
focus of a taphonomic study of fossil invertebrates in the 1980’s. The Purisima
Formation is rather unique in that, unlike most rock units which have received
taphonomic study, it preserves invertebrate and vertebrate fossils in a number
of different depositional environments. This provided Norris (1986) with the
unique opportunity of examining across-shelf trends in preservation of marine
invertebrate fossil concentrations. Even with such an expansive shelly fossil
record, similar studies have been few and far between. No studies investigating
across-shelf trends in marine vertebrate taphonomy had ever really been
attempted. My own limited field experience at the time indicated that a study
of similar scope as Norris’s original paper – but analyzing marine vertebrates
from the Purisima Formation instead – would uniquely permit the examination of
cross-shelf changes in vertebrate preservation. All previous studies had
sampled vertebrates from a single marine unit reflecting a single depositional
environment, or a single fossil bed, or a single skeleton. These studies are of
course necessary and make up the bread and butter of marine vertebrate
taphonomy, but investigating larger scale processes that control or influence
the spatial distribution and preservation of vertebrate bones and teeth in the
marine environment is (or, was) virgin territory.

I’ll discuss the highlights later on in a dedicated series
of posts, but these are takeaway points:

3) this indicates a systematic relationship between
sedimentary architecture and marine vertebrate preservation, and that the sheer
majority of the marine vertebrate fossil record is controlled by physical
sedimentary processes, rather than biotically controlled. From a
paleoecological perspective, there is not much hope that using any sort of
specimen counting methods (e.g. relative abundance) for faunal analysis will be
able to backstrip the rather severe taphonomic overprint.

Friday, March 7, 2014

In 1976, a strange large bodied shark with a wide mouth and
a multitude of tiny, unicuspate teeth was discovered after being entangled in
an anchor of a US Navy ship off the coast of Hawaii.
Preliminary examination indicated it was an entirely new genus and species of
filter feeding shark, not similar or closely related to basking sharks (Cetorhinus)
and whale sharks (Rhincodon). It was named several years later as Megachasma
pelagios – the megamouth shark. Megachasma is approximately 4-6
meters in length, inhabits temperate waters of the Atlantic, Pacific, and
Indian Oceans, and extraordinarily rare – only 55 specimens have been observed
since its discovery, explaining why this shark took so long to discover (in
contrast, most other large bodied sharks at temperate latitudes have been known
to science since the 18th century).

The modern megamouth shark, Megachasma pelagios.

This week, a new species of fossil megamouth – named Megachasma
applegatei after the late paleoichthyologist Shelton Applegate – was
described by Kenshu Shimada, Bruce Welton, and Doug Long. Fossil teeth of M.
applegatei occur in the late Oligocene-early Miocene Pyramid Hill member of
the Jewett Sand near Bakersfield (California),
the Skooner Gulch Formation in Mendocino
County (California),
and the Yaquina Formation and Nye Mudstone of coastal Oregon.
Oddly enough, despite being named recently, the first fossils of this new
species were discovered (at the Pyramid Hill locality) fifteen years prior to
the discovery of the modern megamouth shark – which sort of makes the modern
megamouth shark a living fossil.

The holotype and some paratypes of Megachasma applegatei.

Shimada et al. (2014) describe in total a series of 67 teeth (see above)
– virtually every specimen present in museum collections. Many more specimens
are present in private collections, but are useless to paleontologists
interested in publishing as they are not publishable specimens. Private
specimens include many published in an earlier study by de Schutter (2009), who
unfortunately published photographs and descriptions of specimens in private
collections. The 67 specimens reported by Shimada et al. (2014) include all
publishable specimens, and constitutes a fairly large sample set. Other
Cenozoic sharks are represented by tens of thousands of specimens – but a fair
amount of variation is recorded in this sample. This large sample demonstrates
two primary morphological differences between Megachasma applegatei and
extant Megachasma pelagios: relatively shorter crowns (relative to root
size) and the primitive retention of lateral cusplets in M. applegatei.
The lateral cusplets and overall morphology of the teeth of M. applegatei
are reminiscent of sand tigers (Odontaspididae), and appear to retain some
primitive lamniform tooth morphology.

The rather large sample size of Megachasma applegatei. Serious kudos to the authors for figuring every single specimen!

The authors also review the rest of the published fossil
record of Megachasma, and demonstrate that most Cenozoic teeth fall into
two categories: Megachasma applegatei and similar teeth from Belgium
from Mio-Pliocene deposits, and younger Pliocene specimens much more similar to
extant Megachasma pelagios (e.g., Pliocene Yorktown Formation, Lee Creek
Mine, North Carolina). The third
species, Megachasma comanchensis, was described earlier by Shimada
(2007) from the Cretaceous of the western interior (USA)
but has been challenged by other authors as not genuinely representing a
Cretaceous megamouth shark.

This study and two recent papers on fossil basking sharks
mark the return of paleoichthyologist Bruce Welton, who published quite a bit
during the 1970’s and 1980’s, but was less productive prior to his retirement
from the petroleum industry. I’m truly pleased that this paper is finally out,
and am eagerly looking forward to more papers on fossil sharks from the North
Pacific. On that note, I will conclude that I have just submitted my own paper
on fossil sharks from the region – with Dana Ehret, Doug Long, Evan Martin, and
my wife Sarah – so, there will be more to read in the somewhat distant future!

References

De Schutter, P. 2009. The
presence of Megachasma (Chondrichthyes: Lamniformes) in the Neogene of
Belgium, first occurrence in Europe. Geologica
Belgica, 12: 179–203.

Shimada, K., Welton, B.J., and Long, D.J. 2014. A new fossil
megamouth shark (Lamniformes, Megachasmidae) from the Oligocene-Miocene of the
western United States.
Journal of Vertebrate Paleontology 34:281-290.

Monday, March 3, 2014

Last weekend I was interviewed by Graeme Hill for the New Zealand station Radio Live, which broadcasted here over the weekend. For those of you who live elsewhere and probably missed it, you can listen to a podcast [9:55 min], linked below:

The interview covers the results of a recently published paper in Geodiversitas regarding fossil marine mammals from the Pliocene Purisima Formation of California. It covers some of the behind the scenes stuff - the discovery of the fossil site, some details of the ten years of laboratory work involved, in addition to discussing the broader implications of the fossilized fauna, and potential insights into the appearance of modern marine mammal species in the North Pacific.

Number of visits

About the Coastal Paleontologist

I'm a paleontologist and adjunct faculty at College of Charleston in South Carolina, with research interests in Cenozoic marine vertebrates with an emphasis on marine mammals (whales, dolphins, pinnipeds, otters, sea cows, and others), but I willingly entertain brief distractions into the worlds of marine birds, sharks, and fish. My M.S. (2011, MSU-Bozeman) focused on marine vertebrate taphonomy whilst my Ph.D. (2015, U. Otago, NZ) focused on Oligocene baleen whales from New Zealand. Current research is concerned with fossil cetaceans from South Carolina including Oligocene eomysticetids, toothed mysticetes, and archaic dolphins.